The present invention relates to an electrophotographic photosensitive member, and more specifically, to a photosensitive member comprising a dispersion layer and a photosensitive layer sequentially superimposed over a substrate.
One aspect of the present invention relates to an intermediate layer provided between a substrate layer and a photosensitive layer.
Another aspect of the present invention relates to a protective layer provided for a photosensitive layer.
Electrophotographic photosensitive members are generally formed by providing a photosensitive layer over an electrically conductive substrate such as aluminum or the like. When a photosensitive member is constructed by forming a photosensitive layer directly over a conductive substrate, however, unnecessary charge is readily injected from the substrate so as to easily produce noise in the formed image. This charge injection is believed to be caused by a general irregularity of the surface of the conductive substrate made of aluminum or the like. As a result of surface irregularities, a concentration of charge easily occurs at the surface convexities or protrusions so as to cause a breakdown of said convexities. Therefore, for example, in the case of positive developing, the electrostatic latent image required for image formation on the photosensitive member is erased by the injection of unnecessary charge from the substrate. A toner image is not formed in these areas regardless of whether or not these areas are supposed to form the toner image. This results in so-called white spots and image noise. As a further example, in the case of reverse developing, the toner image is conversely formed in areas in which image formation is not supposed to form or occur. This results in so-called black spots and image noise.
An intermediate layer can be formed between the conductive substrate and the photosensitive layer to prevent the injection of unnecessary charge from said conductive substrate.
When such an intermediate layer is formed as an insulation layer, comprising a single resin having a high electrical resistance, a smooth flow of charge from the substrate to the photosensitive layer is impeded. This leads to a separate disadvantage, wherein following optical exposure, the surface potential of the photosensitive member is not reduced to a predetermined value, thereby resulting in an elevation of the residual potential.
Although the electrical resistance can be reduced by making the insulation layer extremely thin in order to eliminate the aforesaid problem, another problem is encountered. In particular, when the layer thickness is made too thin, the defects and irregularities of the conductive substrate surface are not adequately covered, and the function of the insulation layer as an intermediate layer is not sufficiently realized. Furthermore, various types of conductive additives can be included within the insulation layer. For example, Japanese Unexamined Patent Application No. SHO 60144755 discloses a resin dispersion layer containing antimony-doped tin oxide as conductive powder dispersed in resin.
In conjunction with the diversification of electrophotographic apparatuses in recent years, it has become desirable to provide photosensitive members for backside exposure, or belt-like photosensitive members. In connection with such photosensitive members, the use of a conductive intermediate layer formed on a nonconductive substrate of resin film or glass or the like, as a conductive substrate is being investigated.
Relative to another aspect of the present invention, the surface of a photosensitive layer, and particularly the surface of a photosensitive layer of an organic type photosensitive layer, is generally provided with a protective overcoat layer over the photosensitive layer to prevent injury to said photosensitive layer and improve durability. Photosensitive members are repeatedly subjected to charging and image exposure. Therefore, a protective overcoat layer requires low insularization to prevent an accumulation of charge in the interior portion or surface of the protective overcoat layer. When the electrical conductivity is excessively high, charge migration occurs in a horizontal direction and causes the production of unsharp images. Conversely, when conductivity is too low, charge accumulates and causes image fogging. Therefore, the conductivity of the protective overcoat layer must be controlled to a suitable value, and said conductivity must remain stable in the presence of external influences such as temperature and humidity and the like.
The protective overcoat layer must satisfy mechanical strength requirements so as to prevent injury from a toner cleaning blade or the like.
A protective overcoat layer may be colored by material added for low insularization insofar as such material does not produce undesirable affects on spectral sensitivity of the photosensitive member.
From this perspective, a layer having conductive particles dispersed in a binder resin can be used as a protective overcoat layer. Japanese Unexamined Patent Application No. SHO 56138742, for example, discloses a protective overcoat layer comprising a tantalum-doped tin oxide powder as electrically conductive particles dispersed in resin.